The present invention relates to a hydraulic control arrangement for actuating a lifting mechanism of an agricultural machine. More particularly it relates to a hydraulic arrangement for actuating a lifting mechanism of an agricultural machine, particularly a tractor, which has a pressure medium source, a directional control valve for controlling a hydraulic motor which lifts and lowers at least one member coupled with a machine, wherein forces transmitted from the member in a machine housing are at least partially transmitted via a structural element with bending elasticity whose bending is used by a hydromechanic transformer as a control value, and a hydraulic control valve of the transformer controls the directional control valve.
A hydraulic control arrangement of the abovementioned general type is known in the art. Such a control arrangement is disclosed, for example, in a German Offenlegungsschrift No. 3,021,777. The control arrangement disclosed in this reference is used for controlling the pulling force in lower members of a tractor lifting mechanism, wherein a supporting frame with a bending bar is fixedly mounted on the housing of the tractor. The bending of the bending bar is determined as a control value by a pick-up and supplied to a control circuit. Thereby test data indicating jumps are avoided in an advantageous manner because of a play which can take place during load variations of measured pulling and pressing forces in the lower members. This control arrangement possesses however, the disadvantages that it has high structural expenses for a rigid supporting frame with the bending bar. In addition to this, a space required for such a supporting frame with the bending bar is frequently not available in the tractors. Further the deviation of the control value by the deformation of the bending bar only from a portion of the forces in the lower member can disadvantageously affect the accuracy of the control. Moreover, the arrangement of the pick-up at all ends of the bending bar subjected to carrying leads in practice to difficulties, particularly for the signal pick up. For many applications, there is further the disadvantage that in this control arrangement the bending bar can engage only the sum of the lower member forces.
Another hydraulic control arrangement is disclosed in the U.S. Pat. No. 4,085,806 and used for pulling force control in a tractor. The forces acting upon a member is measured via a torsion spring and transformed in a path which then is available via a control linkage for controlling a directional control valve. The disadvantage of this control arrangement is the high expenses, particularly in the sense of the space and material. Moreover, this construction is less suitable for a fully hydraulically operating control arrangement for lifting mechanism adjustment. In addition, this control arrangement works with a relatively great hysteresis.